Interpretive Summary: Photosynthesis is the single most important source of O2 and organic chemical energy necessary to support all non-autotrophic life forms. Plants compartmentalize this elaborate biochemical process within chloroplasts in order to safely harness the power of solar energy and convert it into usable chemical units. Stresses (biotic or abiotic) that challenge the integrity of the plant cell are likely to affect photosynthesis and can be detected by changes in chlorophyll fluorescence. A simple three-step assay was developed as a rapid biomarker to determine their effect on photosynthesis. Compounds that interfered with photosynthesis directly were the most easily detectable, but some of the other herbicides also induced chlorophyll fluorescence although their mode of action were not directly related to photosynthesis.

Technical Abstract:
Photosynthesis is the single most important source of O2 and organic chemical energy necessary to support all non-autotrophic life forms. Plants compartmentalize this elaborate biochemical process within chloroplasts in order to safely harness the power of solar energy and convert it into usable chemical units. Stresses (biotic or abiotic) that challenge the integrity of the plant cell are likely to affect photosynthesis and can be detected by changes in chlorophyll fluorescence. A simple three-step assay was developed to test selected herbicides representative of all the known herbicide mechanisms of action and a number of natural phytotoxins to determine their effect on photosynthesis as measured by chlorophyll fluorescence. The most active compounds were those that interact directly with photosynthesis (inhibitors of photosystem I and II), those that inhibit carotenoid synthesis, and those with a mechanism of action that generates reactive oxygen species and lipid peroxidation (uncouplers and inhibitors of protoporphyrinogen oxidase). Other active compounds targeted lipids (very-long-chain fatty acid synthase and removal of cuticular waxes). Therefore, induced chlorophyll fluorescence is a good biomarker to help identify certain herbicide modes of action and their dependence on light for bioactivity.